Direct Observation of OH Formation and Luminescent Emission from Photoexcited Acetaldoxime

Abstract

On photoexcitation at 193 nm, the 1(π, π*) excited acetaldoxime (CH3−CHN−OH) appears to be undergoing intersystem crossing producing a highly energized triplet state, which is followed by parallel processes of the emission of a UV photon at 310 nm and the dissociation to CH3−CHN and OH radicals as primary products. While the laser-induced fluorescence showed that only 1.5% of the nascent OH (X2Π) is produced in the vibrationally excited state with v = 1, there is no OH produced with v = 2. The rotational state distribution of OH is found to fit a Boltzmann distribution, characterized by a rotational temperature Trot of 1200 ± 120 K for the v = 0 and Trot of 990 ± 100 K for the v = 1 vibrational states, respectively. By measuring the Doppler spectroscopy of the v = 0 and v = 1 states of OH, the translational energy of the photofragments is found to be 40.0 ± 5.0 and 32.2 ± 4.0 kcal mol-1, respectively. While 20 kcal mol-1 of translational energy is expected statistically, imparting such a large amount of translational energy into the products suggests that the dissociation occurs on the excited state potential energy surface. The real time formation of OH shows a dissociation rate of the acetaldoxime to be (1.5 ± 0.3) × 106 s-1. The above dissociation rate vis-à-vis statistical Rice−Ramsperger−Kassel−Marcus theory suggests that the acetaldoxime dissociates from the triplet state, with a threshold dissociation energy of about 49 kcal mol-1. The decay of the triplet acetaldoxime emission at 310 nm with a rate of (1.2 ± 0.3) × 106 s-1, similar to that of its dissociation to form OH, further suggests that both the competitive decay processes occur from the triplet state potential energy surface.